1. Field of the Invention
The invention relates to photothermographic materials that are light sensitive, and in particular, materials free of silver halide which are based on silver soaps that are thermally developable.
2. Information Disclosure Statement
Photothermographic imaging materials based on the chemistry of silver salts of organic acids have been long known. In the earliest examples (Talbot, U.S. Pat. No. 5,171 (1847)) the intrinsic light sensitivity of the silver salt of the acid, e.g., silver acetate, was used to create the latent image, amplified by thermolysis of the silver salt. Later investigators based fundamentally similar systems on silver oxalate (Sheppard and Vanselow, U.S. Pat. No. 1,976,302 (1934); U.S. Pat. No. 2,095,839 (1937); U.S. Pat. No. 2,139,242 (1938); Suchow and Herah, U.S. Pat. No. 2,700,610 (1955)). The early history of photothermography has been reviewed by Klosterboer (in Neblette's Imaging Processes and Materials, Sturge, Walworth and Shepp, eds. (New York, van Nostrand Reinhold, 1989), chap. 9).
Materials with useful photographic speeds have, however, up until now, required the use of silver halide as a light sensitive component (Sorensen and Shepard, U.S. Pat. No. 3,1522,904 (1964; reissued 1969); Yutzy, U.S. Pat. No. 3,392,020 (1968); Morgan and Shely, U.S. Pat. No. 3,457,075 (1969)). A commonly perceived drawback of these compositions is the persistence of photochemical activity of the silver halide after thermal processing of the imaging material. This leads to instability of the processed image on the medium when exposed to light (Kurttila, J. Micrographics, 10: 113 (1977)). It is one purpose of this invention to eliminate the use of silver halide in photothermographic imaging media.
Relatively little literature exists on silver salts of tetrahydrocarbylborate anions (herein "silver organoborates"). Silver tetraphenylborate is easy to prepare by mixing solutions of silver tetrafluoroborate and sodium tetraphenylborate, both in methanol; the product precipitates and can be collected and dried in the usual manner. It is noticeably light sensitive.
It is relevant to the present invention that when a silver iodide dispersion (ca. 450 .ANG. particle size) in 2-butanone is treated with excess tetrabutylammonium n-butyl-triphenylborate, the dispersion shortly ceases to exhibit a Tyndall effect (characteristic of the presence of colloidal particles), and the exciton absorption of hexagonal AgI at 422 nm also disappears from the absorption spectrum; the solution in fact becomes transparent beyond 320 nm. These observations indicate dissolution of the AgI by metathetical conversion to the organoborate salt, which happens to be soluble in 2-butanone.